Transect Scale Solute Transport Measured by Time Domain Reflectometry

Abstract

Two quasi steady-state solute transport experiments were carried out in a loamy sand under field conditions. The flux was 40 mm/d in experiment 1 and 18.7 mm/d in experiment 2. Both water content (θ) and resident concentration (Cr) measurements were taken using 64 time domain reflectometry probes at depths ranging from 0.05 to 0.90 m. The Cr measurement was calibrated in situ for each probe location in the field. The convective dispersive equation (CDE) and convective lognormal transfer function (CLT) models were fitted to the breakthrough curves (BTCs). The results indicated fingered flow, which has been shown to exist in previous studies of this soil. The finger width was larger in experiment 1 leading to smaller horizontal heterogeneity and a relatively smaller solute transport velocity. The location of the fingers was consistent between the two experiments resulting in a high correlation between the velocity and mass balance fields. Mass balance calculations showed that the solute mass integrated over depth one day after the solute application was larger than the mass balance for the entire experiment (integrated over time). The probable reason being that solutes were transported out of the measurement volume by horizontal flow across the Ap/E horizon boundary. The investigation of the transport parameters revealed that both the CDE and CLT models could be successfully used to predict most individual BTCs. Horizontally averaged global CDE and CLT models were also fitted to the data. Global solute transport was better modeled with the CDE model in experiment 1, while in experiment 2, the CLT model was better. This study clearly shows the applicability of using TDR with the in situ calibration technique in field experiments with varying water content.